Fortunately for us, the brain never sleeps, even when the body looks asleep. Otherwise we would never wake up each morning. Here's a summary of what is going on in the body while we sleep:
Light sleep occurs during sleep stage 1 and sleep stage 2.
It is very easy to arouse someone during light sleep.
Getting lots of light sleep does not mean you'll feel rested the next day.
Half the night is normally spent in light sleep, spaced throughout the night.
That might increase to 3/4 of the night if there is a sleep disorder.
Some dreams may occur, usually related to thoughts while falling asleep.
Lots of people who think they never sleep are actually in light sleep.
Especially the ones who say they saw the clock turn every hour.
Expect blood pressure & breathing to be the same as when awake, at rest.
Pulse rate is about the same when awake and at rest - 72bpm on average.
Delta is the deep restorative sleep stage; you won't feel rested without it.
EEG studies show slow brain wave activity during this sleep stage.
Less than 1/2 the night is spent in deep sleep; some in Delta, some in REM.
The breathing rate slows down, but the size of the breath stays the same.
And the oxygen and CO2 levels remain fairly normal (90-100% and 40mm).
Studies show that most Delta sleep is normally in the first 1/3 of the night.
Lack of Delta sleep will greatly increase your odds of getting sick.
Experiencing a groggy feeling is common if awakened during Delta sleep.
Especially if you take too long of a nap.
Pulse rate drops by 5 to 20 beats; blood pressure drops as well.
Dreams commonly happen during this stage as does a type of paralysis.
Respiratory rate gets faster and breaths become more shallow.
Eyes will move rapidly (REM) behind the eye lids during dreams.
And EEG studies show rapid brain activity, similar to when you are awake.
Most people remember dreaming at least now and then.
Studies show that most REM sleep is normally in the last 1/2 of the night.
Levels of O2 may drop 3-5% in healthy lungs; 5-10% in damaged lungs.
Especially if there is also sleep apnea; then levels may drop 30 - 40%.
Equally of concern in damaged lungs is that CO2 levels may be going up.
Pulse rate goes up, as does blood pressure.